What do the bars between keyframes mean?
This means that the keyframes which the bar connects are the same.
From the manual:

When the value of a given channel does not change at all between two neighboring keyframes, a gray (unselected) or yellow (selected) line is drawn between them.

E.g. If you insert a LocRot keyframe on frame 1, then rotate and insert another LocRot keyframe on frame 10, the Location will not have changed so there will be a bar drawn between each location channel keyframe: